A machine that moves by means of magnetic effects mimicking the behavior of a human is called a “robot.” The term robot is said to be derived from Slavic word Robota (slave machine). In Japan, robots started to be widely used from the 1960s, and most of the robots were then industrial robots such as manipulators and conveyance robots for automating a production line in a plant or for use in an unmanned plant.
Research and development have advanced in moving legged mobile robots that imitate the body mechanism and the motion of living things such as humans and apes, which perform bipedal walking, and expectation about the commercialization of these robots is mounted. The legged mobile robot is unstable and presents more difficulty in posture control and legged control than crawling robots, quadrupedalling robots, and sextuple pedalling robots. However, the legged mobile robot is excellent in that the legged mobile robot is flexible in moving over, for instance, bumpy walking surfaces with unregulated surfaces and obstacles, or discontinuous walking surfaces such as stairs or ladders.
The legged mobile robots which mimic the body mechanism and the motion of the human beings are called “human-like figured” or “human-type” robots (humanoid robots). Humanoid robots assist daily life and a variety of human activities in living environments.
The work space and living space of human beings are formed in accordance with the body mechanism and the behavior pattern of bipedal walking human beings. In other words, currently available mechanical systems having wheels or other driving devices as travel means must overcome a diversity of barriers existing in the living space of the human beings. For mechanical systems such as robots to perform various human activities for human beings and become ubiquitous in the living environment, the area of work of the robot is preferably almost identical to that of the human beings. For this reason, the commercialization of the bipedalling legged mobile robots is expected. Bipedal walking capability is an essential requirement for enhancing the intimacy of the robot with the living environment of the human beings.
Numerous techniques have been proposed relating to the posture control and stable walking of the bipedalling legged mobile robot. The stable “walking” may be defined as a “movement around using legs without falling.” The posture control of the robot is important to avoid falling during the operation of the robot. The falling means an interruption of the activity of the robot, and a great deal of labor and time is required to stand up from the fallen state and to resume the activity. There is a risk that the robot itself is damaged and that an object that could be hit by the falling robot may be severely destroyed. In the design and development of the legged mobile robot, the posture control for stable posture and falling prevention during walking are the most important issues.
Many proposals relating to the robot posture control and the robot falling prevention during walking use a ZMP (Zero Moment Point) as a walk stability determination criterion. The stability determination criterion of the ZMP is based on “D'Alembert's principle” according to which gravity and inertia working from a walking system on a walking surface balance a floor reaction force and a floor reaction moment as reaction working from the walking surface to the walking system. As a result of kinetic consideration, a point at which a pitch axis moment and a roll axis moment become zero “ZMP” (Zero Moment Point) is present along the edge of a supporting polygon (namely, a stable ZMP region) formed between the sole contact point and the walking surface or within the supporting polygon.
In summary, the ZMP criterion refers to a rule “At any moment of walking, the ZMP is present within a supporting polygon formed of a foot and a walking surface, and if a force a robot applies on the walking surface works, the robot stably walks without falling (without a rotary motion).”
In accordance with the bipedalling pattern based on the ZMP criterion, a sole landing point is set beforehand, and the kinetic restriction conditions of a toe are easy to consider accounting for the configuration of the walking surface. Since the use of the ZMP as the stability determination criterion means that a course rather than a force is treated as a target in motion control, the technical feasibility of using the ZMP as the stability determination criterion is high.
The concept of the ZMP and the application of the ZMP in the stability determination criterion of the legged mobile robot are described in a book entitled “LEGGED LOCOMOTION ROBOTS” authored by Miomir Vukobratovic (“LEGGED ROBOT AND ARTIFICIAL FOOT” translated by Kato et al. (NIKKAN KOGYO SHINBUN)).
In conventional typical posture control and walk control of the robot using the ZMP as the stability determination criterion, the ZMP position is controlled to return to a stable region if the ZMP position is deviated from the stable ZMP region. In other words, during a normal operation, the ZMP is free to move. The ZMP position is controlled in response by controlling joints such as of a leg after the fact that the amount of momentum exceeds a predetermined limit.
The legged mobile robot disclosed in Japanese Unexamined Patent Application Publication No. 5-305579 is designed to stably walk with a point on a floor having a zero ZMP set to be a target value.
In the legged mobile robot disclosed in Japanese Unexamined Patent Application Publication No. 5-305581, a ZMP is positioned within a supporting polyhedron (a polygon) or during landing on or lifting from a floor, the ZMP is positioned to be within a predetermined margin from the edge of a supporting polygon. In this case, the ZMP has the predetermined margin even under the presence of disturbance, and the stability of the robot body is increased during walking.
Japanese Unexamined Patent Application Publication No. 5-305583 discloses a control method in which a walking speed of a legged mobile robot is controlled according to a ZMP target position. Specifically, leg joints are driven so that the ZMP agrees with the target position, using predetermined walk pattern data, and the inclination of the upper body of the robot is detected and a set walk pattern data output rate is modified in accordance with the detected inclination of the upper body. When the robot is forwardly inclined as a result of stepping on unknown walking surface irregularities, the output rate may be increased to recover the posture of the robot. With the ZMP controlled to the target position, the output rate may be modified without any problem during a double support phase.
Japanese Unexamined Patent Application Publication No. 5-305585 discloses a control method in which a landing position of a legged mobile robot is controlled in accordance with a ZMP target position. Specifically, the disclosed legged mobile robot detects a difference between a ZMP target position and an actually measured position, and one or both of the legs are driven to eliminate the difference, or a moment about the ZMP target position is detected and the legs are then driven to reduce the moment to zero.
Japanese Unexamined Patent Application Publication No. 5-305586 discloses a control method in which an inclined posture of a legged mobile robot is controlled in accordance with a ZMP target position. Specifically, a moment about the ZMP position is detected, and stable walking is achieved by driving a leg unit until a moment becomes zero when the moment takes place.
The above-mentioned attitude control methods of the robot are based on a basic operation in which a point at which a pitch axis moment and a roll axis moment become zero is searched for within or along the side of a supporting polygon formed of the ground contact point of a sole of the movable leg and a walking surface, namely, a stable ZMP region. Correction control is performed to return to a stable ZMP region when the ZMP position is deviated from the stable ZMP region.
The ZMP criterion is merely a criterion which is applicable on the assumption that the body of the robot and the walking surface are extremely close to a solid body in nature (specifically, these are not deformable nor movable even under any force or moment). In other words, when the assumption that the robot body and the walking surface are extremely close to a solid body in nature does not hold, a (translational) force acting on the ZMP with the robot moving at a high speed and an impact at the switching of supporting legs become large, and the robot itself is subject to a momentum. Without adequately managing the momentum of the robot in response to the force exerted on the robot, a space within which the ZMP is present becomes unstable. Even if the posture of the robot satisfies the. ZMP criterion (namely, the ZMP is present within the supporting polygon, and the robot exerts a pressing force on the walking surface), the robot posture itself becomes unstable as a result of stabilizing an unstable ZMP. As the center of gravity of the robot becomes lower in level, a rotary motion takes place in the robot body, and stable walking becomes very difficult to accomplish.
FIG. 1 and FIG. 2 respectively illustrate an ideal model in which a robot and a walking surface are extremely close to a solid body in nature, and the relationship (in other words, a ZMP behavior space of the robot) between the ZMP position and the momentum of the robot when the robot body and the walking surface are really not solid.
In the ideal case in which the robot and the walking surface are extremely close to a solid body in nature, no momentum takes place in the robot in any ZMP position in the calculated stable ZMP region within the ZMP behavior space as shown in FIG. 1. In other words, the stable posture of the robot body is not destroyed at any ZMP position.
In the ZMP behavior space in an actual system, the robot and the walking surface are not solid. Even within the calculated stable ZMP region, a momentum takes place in the robot depending on the ZMP position. In the case illustrated in FIG. 2, no momentum takes place in the robot in the vicinity of the center of the stable ZMP region. In this state, the stable posture of the robot is not destroyed. As the ZMP position outwardly shifts from the center of the stable ZMP region, the momentum of the robot increases in a negative direction.
Referring to FIG. 1 and FIG. 2, the ZMP behavior space is defined by a ZMP position and a floor reaction force of a floor surface acting on the robot body. Concerning the direction of the momentum of the robot, a negative direction is defined as a direction which distorts the space in a manner such that the ZMP position shifts toward the periphery of the stable region, and a positive direction is defined as a direction which distorts the space in a manner such that the ZMP position shifts to the center of the stable region. As shown in FIG. 2, the momentum of the robot increases in the negative direction as the ZMP position outwardly shifts from the center of the stable ZMP region. Even within the stable ZMP region, the robot is displaced toward the periphery of the stable ZMP region, and finally falls.
For this reason, posture control must be continuously performed to shift the ZMP position back to the center of the stable ZMP region even if calculation shows that the ZMP position of the robot stays within the stable ZMP region. A typical example of the control method in which the ZMP position is continuously shifted back to the center is an “inverse pendulum.” In this case, high-speed control needs to be carried out (specifically, the sampling period of the system is extremely short), and the workload on a computer performing posture control increases.
The ZMP stability determination criterion is a mere criterion which is based on the assumption that walking is performed under ideal environments including preconditions that cannot be realized under actual operation environments. To allow the robot to continuously walk in an autonomous fashion under living environments of human beings, a robot system configuration accounting for the stability of the ZMP space needs to be invented.
The stability and the controllability of the legged mobile robot during a legged job are subject to not only a gait pattern or a motion pattern of the four limbs but also conditions of the ground and walking surfaces on which the legged job such as walking is performed. This is because as long as the legs of the robot are in contact with the walking surface, the robot is continuously under a reaction force. The reaction force from the walking surface becomes a large impact particularly when the lifted leg lands on the ground during the legged job such as walking. The reaction force can become a disturbance, leading to an instability of the robot.
In other words, to allow the biped walking robot to perform the legged job without destroying the posture thereof, the robot preferably fits the ground with a stable posture thereof kept at the moment of landing, and the reaction force of the walking surface acting on the robot during the landing is preferably reduced as much as possible. The structure of the sole of the foot which comes into contact with the ground is extremely important to establish an excellent relationship between the robot and the ground surface.
To absorb the impact from which the robot suffers from the ground surface during the landing, it is well known to those skilled in the art that an elastic material is applied on the sole of the foot.
When the robot receives a reaction force from the ground surface during the landing of the leg, the robot body is subject to various disturbances about the roll axis and the pitch axis thereof. The legged mobile robot which mainly moves forward has a wide degree of freedom in the fore-aft direction, and relatively easily responds to disturbances in the forward direction, namely, about the pitch axis. On the other hand, the legged mobile robot has a low robustness to disturbances about the roll axis that roll the robot. Even if the impact from the ground surface is uniformly absorbed by the elastic material on the sole of the foot, the impact about each axis is merely absorbed. The robot stable posture which has been destroyed at the landing of the foot or by other disturbances is not restored. Although the impact at the moment of landing is absorbed, the robot body may finally fall. In this case, posterior control in which the ZMP is corrected after the fact that the ZMP position has been deviated from the stable ZMP region is subject to limitation.
The search for the ZMP means the passage of the trajectory of the ZMP between both right and left feet (namely, the inside of each foot) in the bipedalling legged mobile robot body. More specifically, when the ZMP trajectory is shifted outside of one foot as a result of the movement of the robot, the stable posture of the robot cannot be maintained unless the robot steps on the outside of the other foot. This operation is a crossing operation of the legs across the one foot and the other foot, and is a physically and mechanically difficult action in which the right and left legs interfere with each other.
The biped walking robot is designed based on the movement in accordance with the behavior mechanism of the human beings and apes. The robustness of the robot to disturbances in the fore-aft direction is high, while the robustness to disturbances in the traverse direction is relatively low.
In view of the computation speed of a controller, the response speed of a target to be controlled, and other parameters, it is extremely difficult to control the posture of the robot by operation control alone during the legged job. In other words, the posterior control in which the ZMP is corrected after the fact that the ZMP position has been deviated from the stable ZMP region is subject to limitation.